Information on reference signal structure for neighbouring cell measurements

ABSTRACT

The present invention relates to cellular radio communication and in particular to providing information on neighbour cells to enable terminals to perform neighbour cell measurements. In the prior art the terminal attempts to make neighbour cell measurements in a reference signal structure that is the same in the neighbour cell as in the cell the terminal camps in. The present invention is based on the insight that the reference signal structure may differ between neighbouring cell for example in the situation of an MBSFN area that is restricted to a region of all cells of a radio network, or in the situation of TDD mode being applied there may be different regions with different allocation of sub-frames for transmission in the uplink and downlink directions. The present invention solves the problem by broadcast information in a cell indicative of the reference signal structure in neighbour cells.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/595,304, filed Oct. 9, 2009, which is a national stageapplication of PCT/SE2008/050388, filed Apr. 3, 2008, which claims thebenefit of Swedish Patent Application No. 0700900-4, filed Apr. 11,2007, the disclosures of each of which are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention relates to cellular radio communication and inparticular to provide information to mobile terminals that enable theterminals to carry out measurements on cells neighbouring a cell theterminals camp in. The invention also relates to a radio base stationadapted for providing the information, to a mobile terminal and a methodfor the mobile terminal.

BACKGROUND

In the forthcoming evolution of the mobile cellular standards like GSMand WCDMA, new transmission techniques like OFDM are likely to occur.Furthermore, in order to have a smooth migration from the existingcellular systems to the new high capacity high data rate system inexisting radio spectrum, the new system has to be able to operate in aflexible bandwidth. An example of such a new flexible cellular system is3G Long Term Evolution (3G LTE) that can be seen as an evolution of the3G WCDMA standard. This system will use OFDM as the downlinktransmission scheme and will be able to operate on bandwidths rangingfrom 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will besupported on the largest bandwidth. LTE will support both FDD and TDD asuplink/downlink duplexing schemes. Furthermore, LTE will also supportmulticast/broadcast services (MBSFN) on the same carrier as unicastdata.

An essential part in any cellular system is support of mobility, i.e.,the possibility to move the connection between the terminal and thenetwork from one cell to another cell. To support this, neighboring cellmeasurements are used. While the connection is maintained in a servingcell, the terminal measures on some well defined signal in neighboringcells and reports the measurement result to the network. The network canthen make a decision, for example based on a signal-to-noise ratiomeasurement made by the terminal, whether the connection should be movedfrom the serving cell to a new cell.

In order to carry out downlink coherent demodulation, the mobileterminal needs estimates of the downlink channel. A straightforward wayto enable channel estimation in case of OFDM transmission is to insertknown reference symbols into the OFDM time-frequency grid. In LTE, thesereference symbols are jointly referred to as the LTE downlink referencesignals.

FIG. 1 is grid in the time frequency domain, with each square in thegrid representing one subcarrier of one OFDM symbol. It serves todemonstrate the LTE downlink reference-signal structure assuming normalcyclic prefix, i.e. seven OFDM symbols per slot. As illustrated in FIG.1, downlink reference symbols are inserted within the first and thethird last OFDM symbol of each slot and with a frequency-domain spacingof six subcarriers. Furthermore, there is a frequency-domain staggeringof three subcarriers between the first and second reference symbols.Within each resource block, consisting of twelve subcarriers during oneslot, there are thus four reference symbols. This is true for allsub-frames except sub-frames used for MBSFN-based broadcast/multicast,see further below.

The structure in FIG. 1 illustrates the reference-signal structure forthe case of a single antenna. For various multi-antenna transmissiontechniques, there is typically one reference signal transmitted for eachantenna (the term ‘antenna port’ is used in the 3GPP specifications) andthe location of the reference signals for the different antennas may bedifferent.

The reference signals can also be used for other purposes than coherentdemodulation. One such example is neighboring cell measurements formobility, where the terminal measures on the reference signal inneighboring cells to support mobility as described above.

One important part of the LTE requirements in terms of spectrumflexibility is the possibility to deploy LTE-based radio-access in bothpaired and unpaired spectrum, i.e., LTE should support both FDD- andTDD-based duplex arrangements. Frequency Division Duplex (FDD) asillustrated in the left part of FIG. 2, implies that downlink and uplinktransmission take place in different, sufficiently separated, frequencybands. Time Division Duplex (TDD), as illustrated in the right part ofFIG. 2, implies that downlink and uplink transmission take place indifferent, non-overlapping time slots. Thus, TDD can operate in unpairedspectrum, whereas FDD requires paired spectrum.

To support TDD operation, a guard time between downlink and uplinktimeslots is needed. This can be created by omitting one or several OFDMsymbols (“puncturing”) in the last sub-frame before thedownlink-to-uplink switch. In case a long guard time is needed, some ofthe reference symbols may need to be punctured in the last sub-frameprior to the switchpoint. The non-punctured part of a subframe used fordownlink transmission is sometimes referred to as DwPTS.

In case of TDD operation, uplink and downlink transmission activityshould be coordinated between neighboring cells. If this is not done,uplink transmission in one cell may interfere with downlink transmissionin the neighboring cell (and vice versa) as illustrated in FIG. 3.Related to measurements, the terminal should only make neighbouring cellmeasurements during downlink transmission slots.

Multi-cell broadcast implies transmission of the same information frommultiple cells. By exploiting this at the terminal, effectively usingsignal power from multiple cell sites at the detection, a substantialimprovement in coverage, or in higher broadcast data rates, can beachieved. In LTE, this is implemented by transmitting not only identicalsignals from multiple cell sites, with identical coding and modulation,but also synchronize the transmission timing between the cells, thesignal at the mobile terminal will appear exactly as a signaltransmitted from a single cell site and subject to multi-pathpropagation. Due to the OFDM robustness to multi-path propagation, suchmulti-cell transmission, also referred to as Multicast-Broadcast SingleFrequency Network (MBSFN) transmission, will then not only improve thereceived signal strength but also eliminate the inter-cell interference.Thus, with OFDM, multi-cell broadcast/multicast capacity may eventuallyonly be limited by noise and can then, in case of small cells, reachextremely high values.

It should also be noted that the use of MBSFN transmission formulti-cell broadcast/multicast assumes the use of tight synchronizationand time alignment of the signals transmitted from different cell sites.

For MBSFN, a different reference signal structure is used as illustratedin FIG. 4. This is needed as the effective channel seen by the terminalin case of MBSFN transmission appears as more frequency-selective than asingle-cell unicast transmission. Thus, as unicast data and MBSFNtransmissions are time multiplexed in different time slots, thereference signal structure will differ between slots in case of a mixedcarrier transmitting both unicast and MBSFN services. In MBSFNsub-frames, only part of the cell-specific reference signal is present,it occurs in some the first OFDM symbols of the sub-frame as disclosedin FIG. 4. The OFDM symbols carrying cell-specific reference symbol inthe MBSFN sub-frame is a sub-set of the symbols used in a normalsub-frame for carrying cell-specific reference symbols, as can beconcluded by comparing FIG. 1 and FIG. 4.

Typically, the terminal assumes the same configuration in theneighboring cell as the current cell. In case neighboring cells areconfigured differently, e.g., different guard times are used inneighboring cells or the MBSFN sub-frame are allocated differently inneighboring cells, the measurements made in the terminal would notcorrectly reflect the situation.

SUMMARY

The present invention solves the above problem by a method ofbroadcasting information in a first cell indicative of a sub-framespecific reference signal structure of the neighbouring cells. Thepurpose of the broadcast information is to enable terminals in the firstcell to perform neighbour cell measurements.

The invention also relates to a method for a mobile station thatmeasures the reference signal of a neighbouring cell assuming it has thephysical structure indicated by broadcasting in the cell the mobileterminal camps in. The invention also relates to a mobile stationadapted for performing the method.

The invention also relates to a radio base station adapted forperforming the method.

In one embodiment, TDD access mode is used, and not all neighbour cellshave the same allocation of uplink and respectively downlinktransmissions in various sub-frames. According to the first embodimentof the invention, only sub-frames used for downlink transmissions by allcells neighbouring the first cell are informed of in broadcasting.Alternatively the downlink sub-frames used by respective neighbour cellis indicated by the first cell.

In a second alternative embodiment, some sub-frame/s is/are used formulti-broadcast transmission in one or more of the neighbouring cells,whereas some other of the neighbour cells or the first cell transmit/s anormal sub-frame structure in the same time window. The multi-broadcastsub-frame has another more restricted allocation of symbols for the cellspecific reference signal than for the normal sub-frame referencestructure. In the second embodiment, is broadcast a structure of asub-set of symbols allocated to symbols in common for both themulti-broadcast sub-frame and for the normal sub-frame. Thereby theterminal will use only the subset of symbols for measures on thereference signal in the neighbour cells.

An advantage of the invention is a terminal will make attempts to detectreference signals only in symbols that are carrying the referencesignal. Thereby it is possible to have different regions of cells withinwhich the same multicast information is broadcast, or within which thesame allocation of sub-frames for respectively uplink and downlinktransmissions is used. The present invention is particular needed incells bordering the different regions of cells, to enable the terminalsto accurately detect the reference signals of all the neighbour cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a grid in the time frequency domain, with each square in thegrid representing an OFDM symbol.

FIG. 2 is an illustration of FDD mode versus TDD mode, with therespective allocation of time and frequency to the uplink and downlinktransmissions

FIG. 3 is a view of two cells and a terminal in each cell illustrating aTDD mode interference situation.

FIG. 4 is the same type of figure as FIG. 1, illustrating a differentallocation of symbols for carrying the reference signal.

FIG. 5 is a view of cells.

FIG. 6 is a flowchart of method.

FIG. 7 is an illustration of different allocation of sub-frames forrespective uplink and downlink transmissions in three cells.

FIG. 8 is a block diagram of a radio base station.

FIG. 9 is a flowchart of a method for a mobile station.

FIG. 10 is a block diagram of a mobile terminal.

DETAILED DESCRIPTION

For neighbour cell measurements the cell specific reference signal shallbe used. Informing of the reference signal structure to be used forneighbouring cells measurement separately from the configuration in theserving cell provides several benefits.

MBSFN transmissions in neighboring cells may use different sub-frameswithout affecting the possibility to perform accurate neighboring cellmeasurements. Although using time aligned transmissions for MBSFN isuseful in case the full benefits of MBSFN are to be exploited, at theborder between different MBSFN regions this is not the case. Theinvention makes it possible to set up different regions of broadcastingthe same MBSFN data. Within a region the same sub-frame allocation isused for broadcasting the MBSFN data. Thereby the broadcasting ofinformation of interest only in a specific geographical area can berestricted to that area. FIG. 5 illustrates a number of cells spread inthe geography, some of them included in a MBSFN region. A first cellneighbouring the MBSFN region, transmits normal sub-frames in the sametime windows as is used in the MBSFN region for MBSFN sub-frames. Alsoother cells neighbouring the first cell transmit normal sub-frames inthe same time window. The physical structure of the normal framereference signals is disclosed in FIG. 1. The reference signal structureof MBSFN sub-frames is disclosed in FIG. 4. The symbols allocated tocarry the reference signal in the MBSFN sub-frame overlap in theirsequential position and on the frequency sub-carrier with symbols usedfor carrying reference signals in the normal sub-frames. The first cellbroadcast information on the reference signal being carried by thesymbols as in the structure of MBSFN broadcasting. Terminals camping inthe first cell will then attempt to detect the cell specific referencesignals according to the structure of the reference signal physicalstructure in the MBSFN sub-frame for all neighbouring cells albeit someof them transmit normal sub-frames. The symbols that carry cell specificreference signal in a MBSFN sub-frame are also used in a normalsub-frame for the same purpose.

FIG. 6 is a flowchart of the method of broadcasting in the first cell,according to the two steps:

-   -   Broadcasting a first reference signal, and that has a first        physical structure (S1). In the situation described as an        example the first cell is not included in the MBSFN region and        the first reference signal structure is the structure of the        normal sub-frame.    -   Broadcasting information indicative of a second reference signal        structure and that is to be used by terminals for performing        neighbour cell measurements.

An alternative embodiment relates to a TDD access mode system when thefirst cell is located between two regions applying different allocationsof sub-frames for respectively UL and DL transmission. A terminal canmeasure on a neighbour cell only when DL transmission direction isapplied in the neighbour cell. When TDD access mode is applied,coordinating the UL and DL transmissions to the same sub-frames in allthe cells of a region, and to synchronize the transmissions in theregion is advantageous because interference between base stations andbetween terminals is mitigated. However, there might be a need to differthe balance of allocation of UL resources versus DL resources inresponse to different demand in different regions. Differentuplink-downlink allocations can be used in different cells, assumingproper planning. This is illustrated in FIG. 7, with a three cells,wherein cell No. 1 and cell No. 3 belongs to two different regions withcell no. 2 in between. The three cells have different allocations ofsub-frames for respective UL and DL transmission directions. Cell No. 1to the left have 8 of its 10 sub-frames dedicated for the DL, and theyare have the sequential numbers 0,1,2,3,5,6,7,8, leaving the remainingtwo sub-frames No. 4 and 9 for the UL direction. A cell No. 2 in themiddle of FIG. 5, has 5 sub-frames allocated for the DL direction, i.e.sub-frames numbered 0,1,2,3,5. Two sub-frames are allocated for the ULdirection, they are numbered 4 and 9. Three sub-frames, No. 6-8 are leftunused. A cell No. 3, to the right in FIG. 5, has 5 sub-frames each tothe UL and to the DL directions. The DL sub-frames are allocated tosub-frames numbers 0,1,2,3,5 and the remaining sub-frames are allocatedto the UL direction. In this case, there is a mechanism to reservesub-frames to one of the three purposes “uplink”, “downlink”, or“unused” in contrast to the, in conventional TDD systems, allocation toone of “uplink” or “downlink”.

In cell No. 2, the broadcast information on which sub-frames to use forperforming neighbour cell measurements, would, in a first alternative berestricted to sub-frames no. 0,1,2,3,5 because this group is common forboth neighbour cell No. 1 and neighbour cell No. 3. Also broadcastinformation in cell No. 1, and cell No. 3 for measurements on respectiveneighbours should be restricted to the use of sub-frames No. 0,1,2,3,5.

In an alternative embodiment the information broadcast by is not onlysub-frame specific but also neighbour cell specific. Thus, cell No. 2broadcast that sub-frames No. 0,1,2,3,5,6,7,8 are available formeasurements on cell No. 1, whereas for measurements on cell No. 3 onlysub-frames 0,1,2,3,5 may be used. This alternative embodiment is alsodisclosed in the text of FIG. 7.

Also in the case of MBSFN transmission, the sub-frame structure isneighbour cell specific. The information on the reference signalstructure is alternatively cell specific for the various neighbourcells.

In the TDD mode, some of the symbols normally used for carrying thereference signals may be punctured for increasing the Guard periodbetween sub-frames for DL to UL transmission. The last reference symbolsin the last DL sub-frame may then be lost. The first cell shall thenbroadcast information of the reference signal physical structure in thesame way as is described for the MBSFN embodiment.

It should further be noted that MBSFN transmission may occur both in FDDmode and in TDD mode. In the case of TDD mode information on whatsub-frames to use as well as physical reference signal structure withineach used sub-frame need be broadcast.

The information indicative of the reference signal structure, need notrelate to the all the symbols carrying the reference signals or to allsub-frames used in DL, it may be restricted to sub-frames or to symbolsthat shall be used by the mobile terminal for making neighbour cellmeasurements. This is in particular relevant to the embodiments when theinformation is indicative of the smallest subset of sub-frames or ofsymbols that are used by all neighbour cells.

Moreover, the broadcast information only need be indicative of aphysical structure. For example different classes of physical structuresmay have been predefined, and the first cell just broadcast theclassification of the different sub-frames.

FIG. 8 is a block diagram of a radio base station adapted for performingthe invention. The radio base station comprises a radio transceiverincluding an antenna system, and a data processor controlling theoperation of the radio base station according to software. The softwareis updated to control the broadcasting including an indication of thesecond reference signal according to the method. The radio base stationalso comprises an X2 interface for connection to neighbouring radio basestation. The signalling information received via the X2 interface isdetected by the data processor and the indication of the secondreference signal structure as broadcast can be updated autonomously bythe radio base station in accordance with information received via theX2 interface.

FIG. 9 is a flowchart of the steps performed by a mobile terminal, or UE(User Equipment) as is the name of the LTE mobile terminal. Initiallythe mobile is camping in a first cell, either in connected mode with thefirst cell acting serving cell or in idle mode. The first cell transmitsa cell specific reference signal according to a first physicalstructure. In the first step, 91, the mobile terminal reads indicationon a second physical structure for a reference signal. The indication isbroadcast in the first cell. In a next step, 92, the terminal measures areference signal in one or more neighbouring cells, assuming thereference signal is carried by symbols according to the indicated secondphysical structure.

The indication of second physical structure typically indicates one in apredefined a set of physical structures. The mobile terminal asmanufactured or as including a SIM-card (Subscription Identity Module),possesses information on the set of possible physical structures for thereference signal.

FIG. 10 is a block diagram of the mobile terminal. It comprises atransceiver, including an antenna, and computer including software. Thecomputer and software may be distributed and part of it residing in theSIM-card. The software is adapted for detecting the broadcast indicationof a the second physical structure of the reference signal, and tocontrol the measuring of reference signal in neighbouring cells to bemade on the symbols that is indicated to carry the reference signal.

So far the invention has been described in a LTE system applying asingle antenna. The LTE is also standardised for alternativemulti-antenna transmission techniques and then typically one referencesignal is transmitted for each antenna and the physical structure ofsymbols assigned for carrying the respective reference signal within thesub-frames, is almost the same. The reference signal structure ofantenna 1 is the same as the reference signal for antenna 0 with, theexception of the OFDM symbol is shifted 3 sub-carriers in the frequencydomain relative to the symbols on carrier 0. The frame timing on the twoantennas is synchronized.

The physical structure of the plural reference signals allocated tomulti-antenna cells, is one of the predefined sets of physical structurethe mobile terminal possesses information on. Accordingly themulti-antenna system physical structure may be broadcast in a cell, andthe mobile terminals assume this physical structure when makingneighbour cell measurements.

Abbreviations:

DL downlink

UL uplink

UE User Equipment, i.e. the name of the mobile terminal in the LTEsystem

MBSFN—a 3GPP specific term used for multibroadcast, i.e. synchronizedbroadcasting of the same information in a plurality of cells. In someliterature this is also referred to as Single-Frequency Network (SFN).

What is claimed is:
 1. A method of broadcasting downlink in a firstcell, comprising: broadcasting a first reference signal, carried by afirst physical structure that comprises normal sub-frames that areallocated to time slots, the first physical structure further involvingthe first reference signal being carried by predefined OFDM symbols andby predefined sub-carriers within the sub-frames; and broadcastinginformation indicative of a second physical structure for carrying asecond reference signal in a neighbouring cell.
 2. The method accordingto claim 1, wherein the first physical structure also comprises MBSFNsub-frames, wherein the predefined OFDM symbols differ between thenormal sub-frame and the MBSFN sub-frame.
 3. The method according toclaim 2, wherein the information indicative of the second physicalstructure relates to the allocation of time slots for carrying the MBSFNsub-frames.
 4. The method according to claim 3, wherein the informationindicative of the second physical structure is indicative of whichsymbols in each sub-frame to use for neighbour cell measurements.
 5. Themethod according to claim 4, wherein the symbols carrying the firstreference signal are allocated to one or more of a set of sub-carriersand to one or more predefined positions in a sequence of symbols withineach sub-frame.
 6. The method according to claim 4, wherein theinformation indicative of the second physical structure is onlyindicative of those symbols used by all neighbour cells of the firstcell for carrying reference signals in each sub-frame for the purpose ofreference signal measurements.
 7. The method according to claim 1,wherein the information indicative of the second physical structurerelates to the allocation of time slots for a downlink transmissiondirection.
 8. The method according to claim 7, wherein the informationindicative of the second physical structure includes information onwhich of the sub-frames to use for the neighbour cell measurements. 9.The method according to claim 8, wherein the one or more sub-frames touse for the neighbour cell measurements are sub-frames used for downlinktransmissions by all neighbour cells of the first cell.
 10. The methodaccording to claim 1 wherein said information indicative of the secondphysical structure is for use by mobile terminals to perform neighbourcell measurements.
 11. The method according to claim 1, wherein thesecond physical structure is indicated by a sub-frame classification.12. The method according to claim 1, wherein the first and the secondreference signal are specific for the cell they are broadcast in. 13.The method according to claim 1, wherein one or more of the time slotsare not used for the allocation in a downlink transmission direction.14. A radio base station comprising: a transceiver arranged to broadcasta first reference signal that has a physical structure that comprisenormal sub-frames allocated to time slots, the first physical structurefurther involving the first reference signal being carried by predefinedOFDM symbols; wherein the transceiver is further arranged to broadcastinformation indicative of a second physical structure for carrying asecond reference signal in a neighbouring cell.
 15. The radio basestation of claim 14, further comprising a data processor that isarranged to control the information indicative of the second physicalstructure to relate to the allocation of time slots for a downlinktransmission direction.
 16. The radio base station of claim 14, whereinthe first physical structure also comprises MBSFN sub-frames, whereinthe predefined OFDM symbols differ between the normal sub-frame and theMBSFN sub-frame.
 17. The radio base station of claim 16, furthercomprising a data processor that is arranged to control the informationindicative of the second physical structure to relate to the allocationof time slots for carrying the MBSFN sub-frames.
 18. The radio basestation of claim 14, further comprising an interface arranged to connectthe radio base station to external units for receiving information onthe second physical structure and that is connected to a data processorof the radio base station.
 19. The radio base station of claim 14,wherein one or more of the time slots are not used for the allocation ina downlink transmission direction.
 20. A method implemented by aterminal, the method comprising: camping in a first cell which istransmitting a first reference signal carried in a first physicalstructure, the first physical structure involving the first referencesignal being carried by predefined OFDM symbols; detecting a broadcastindication of a second physical structure that carries a secondreference signal in a cell neighbouring the first cell, wherein theindication relates to the allocation of sub-frames in a downlinkdirection; and measuring the second reference signal in downlinksub-frames based on the detected indication.
 21. The method of claim 20,wherein the detected indication further relates to OFDM symbols carryingthe second reference signal and wherein the measuring on the secondreference signal is made on the OFDM symbols as indicated.
 22. A mobileterminal comprising a transceiver and a computer, wherein the computerwith software and via the transceiver is operable to: camp the mobileterminal in a first cell which is transmitting a first reference signalcarried in a first physical structure, the first physical structureinvolving the first reference signal being carried by predefined OFDMsymbols; detect an indication that is broadcast in the first cell,wherein the indication relates to the allocation of sub-frames in adownlink direction for carrying a second reference signal in a cellneighbouring the first cell; and measure the second reference signal inthe downlink direction sub-frames based on the detected indication. 23.The mobile terminal of claim 22, wherein the detected indication furtherrelates to OFDM symbols carrying the second reference signal and whereinthe measuring on the second reference signal is made on the OFDM symbolsas indicated.
 24. The method of claim 20, wherein the first and thesecond reference signals are specific to the cell they are transmittedin.
 25. The mobile terminal of claim 22, wherein the first and thesecond reference signals are specific to the cell they are transmittedin.